1. Field of the Invention
This invention relates to a method of detecting a light beam, and more particularly to a method of detecting a synchronizing light beam in a light beam scanning system.
2. Description of the Prior Art
In a light beam scanning system including a light deflector such as a rotating polygonal mirror for making lines on a recording material or the like, a synchronizing light beam detecting means is provided for eliminating a jitter in the lines. In a light beam scanning system which records or displays a figure or characters on a recording material or a cathode ray tube, a jitter in the direction of lines should be removed in order to obtain undeformed figures or characters. The jitter is caused by errors in parallelism of the facets of the polygonal mirror, errors in angles of the facets or displacement of the axis of rotation of the rotating mirror from the center thereof. In order to prevent the jitter, it has been known in the art to provide a light beam detecting means in the line of the scanning light beam to obtain a synchronizing signal. The start of the modulation in each line is put into synchronization with the synchronizing signal obtained by the light beam detecting means.
In the above described light beam scanning system in which a light beam detecting means is provided for synchronization, a photodetector which has a definite size of a light receiving aperture is used as the light beam detecting means. The light receiving aperture is positioned on the focal plane of the light scanning system where the light beam is most concentrated in order to enjoy the highest resolving power. The diameter of the focused light beam on the focal plane is about 10 to 100 microns. On the other hand, the surface of the light receiving portion of the photodetector has surface irregularities and carries dusts or the like thereon. Therefore, there is sometimes observed a jitter in the lines which results from the surface irregularities or dusts on the light receiving portion of the photodetector.